This invention is directed to space heating furnaces, and is more particularly concerned with induced combustion type two-speed gas furnaces.
In a conventional furnace for heating of a residential or commercial space, a thermostat senses that the temperature of an interior comfort space is below a set temperature, and closes to call for heat. The call for heat applies thermostat voltage to the furnace to turn on a gas burner, and, after a predetermined blower-on delay time, to turn on the circulation air blower. The gas burner, which operates at a single rate, injects flame and heated gas into a heat exchanger, which heats the circulation air that is then returned to the interior space. An induced combustion fan draws combustion gases through the heat exchanger and exhausts them into a vent pipe for discharge to the outside environment. Heating continues until the thermostat senses that the interior room air has been heated above the set point, at which time it opens and ends the call for heat.
More recently, two-speed furnaces have been proposed. These have gas burners which can operate at a full flow rate or high fire and can also operate at partial flow rate or low fire. The high fire mode is employed when there is a high demand for heating, that is, when the differences between interior temperature and set point temperature is relatively great. The low fire mode is employed when there is a lower demand for heat, that is when the difference between interior temperature and set point temperature is relatively small. A logic unit, such as a pre-programmed microprocessor, can be employed to actuate the burners in the low- or high-fire modes.
The gas burners can be actuated into the low fire and high fire modes in response to a low-fire combustion pressure switch and a high-fire combustion pressure switch, which sense the negative pressure in the furnace combustion chamber. These serve to turn the burners on only if the inducer fan is bringing enough combustion air in to support the low heat or high heat burner mode. The inducer fan can employ a two-speed motor, typically a two-speed shaded-pole or permanent-split-capacitor ac motor.
Under varying conditions of electrical voltage, and depending on vent design, changing atmospheric conditions, and other variables, the low speed operation of the inducer motor can create enough negative pressure to also actuate the high-fire combustion pressure switch. When this switch is energized, the furnace gas burners go to the high heat mode. However, the inducer and blower remain at their lower speeds. Thus, this produces an non-optimal, and possibly unreliable condition.